Dicarboxylic acids have a high economic potential, since they can be used as precursor substances for numerous chemicals. For instance, succinic acid serves as a preliminary stage for the production of plastics based on 1,4-butanediol, tetrahydrofuran and gamma-butyrolactone. Today, succinic acid is produced chemically by catalytic hydration of maleic acid anhydride to succinic acid anhydride and subsequent water addition or by direct catalytic hydration of maleic acid.
Succinic acid is also formed by many microorganisms from sugars or amino acids under physiological environmental conditions. Under anaerobic conditions, normally further fermentation end products such as ethanol, lactic acid, acetic acid and formic acid are formed, beside succinic acid. The biosynthesis of succinic acid with its high oxygen content requires a reductive CO2 fixation.
Succinic acid is a metabolite, which is normally enriched by anaerobic fermentation processes. Whereas the yield and enrichment of the product under anaerobic conditions is manifold better than under aerobic conditions, the drawback of an exclusively anaerobic process is the technical limitation of the biomass production and a low productivity of the microbial producer. Thus, the consequence is a relatively low biomass/product efficiency. Further, it is difficult to technically handle strictly anaerobic microorganisms.
Various microorganisms that are capable to synthesize succinic acid under anaerobic conditions are known in the art. Document U.S. Pat. No. 5,143,834 describes a variant of A. succiniciproducens. This is an obligate anaerobic microorganism, which can produce moderate amounts of succinic acid only, and is further not capable to tolerate high osmotic pressures and salt concentrations. Document U.S. Pat. No. 7,063,968 describes a microbial rumen isolate, Mannheimia sp. 55E, which is capable to synthesize organic acids under aerobic as well as anaerobic conditions. This is however not a specific enrichment of succinic acid, but a mixture of different organic acids, such as formic acid, acetic acid, lactic acid, and succinic acid. The drawback of this producer is that an economic use of the strain is only difficultly possible, if not impossible, since for obtaining succinic acid, expensive enrichment and purification methods have to be employed. Document U.S. Pat. No. 5,869,301 describes a method for the production of dicarboxylic acids in a two-step fermentation process with E. coli AFP-111, wherein in the first phase microbial biomass is produced under aerobic conditions, and in a second phase the production of succinic acid is carried out in an anaerobic manner. The first phase of biomass generation is limited in this process, since the glucose concentration in the fed-batch process must be limited to 1 g/L, in order to avoid an enrichment of acetate, which disturbs the biomass production process as well as the succinic acid production. Thus, the biomass generation by this process is possible to a limited degree only. Further, the biosynthesis pathway for the succinic acid in this process is subject to a strong catabolite repression, since genes of the glycolysis, of the citric acid cycle and of the glyoxylate pathway are strongly repressed by glucose, as is known from the document DeRisi J. L., Iyer V. R., Brown P. O., Science 278 (5338): 680-686 (1997). The consequence is that the synthesis of succinic acid in presence of glucose is to a great extent repressed and thus strongly limited.
From document U.S. Pat. No. 6,190,914, microorganisms are known in the art, wherein by modulation of suitable transcription factors and kinases the glucose repression of various genes is reduced. The production of organic acids by means of such microorganisms, in particular also further for the production of microorganisms optimized to organic acids, cannot be found therein.